1. Field of the Invention
This invention relates generally to an apparatus and method for calibrating measurement instruments of various types, and in particular to a disposable calibration device and method which uses that device for calibrating measurement instruments that perform measurements on a material or tissue. The calibration device includes a calibration target that ensures proper calibration of the measurement instrument, prevents scratching of windows through which measurements are taken, and also prevents reuse of the disposable calibration target, thereby helping to control the spread of infection if measurements are on tissues, and helping to prevent contamination if measurements are on materials.
2. Background of the Related Art
Many measurement systems require that calibrations be performed on a routine basis in order to compensate for changes in instrument performance and response. This is true for both radiation based measurement systems, i.e., systems that send electromagnetic radiation to the tissue or material to be measured and then detect the return radiation, and acoustic based measurement systems, i.e., systems that send acoustic waves or energy to the tissue or material to be measured and then detect the return acoustic signal. The calibration techniques in both cases typically involve measuring the response of a test target which has characteristics that remain stable with time and over a range of temperatures. Those techniques can also be used to compensate for instrument to instrument variations and any changes that an individual instrument may experience over its working lifetime. Often such measurement systems must be periodically calibrated and sometimes must be calibrated prior to each and every use. This calibration becomes especially important when measurements are made for medical or other critical applications.
Radiation measuring systems are currently used for a wide variety of purposes including to evaluate tissue or materials. These measuring systems require calibration for a variety of reasons including variations in the radiation source intensity, changes in spectral characteristics of the tissue or material, component aging and cleanliness, changes in temperature, radiation detector sensitivity changes, and electronic drifting.
Examples of radiation type measurement systems that often require some type of calibration include but are not limited to spectrometers, laser radar, radar or any other radiation measuring instrument that outputs radiation to a tissue or material and then measures some aspect of the return signal.
Acoustic type measuring systems are also used for a wide variety of purposes including to evaluate tissue or materials. Often these measurement systems must also be periodically calibrated and sometime must be calibrated prior to each use. Acoustic measurement systems also require calibration for a variety of reasons including variations in the output energy of the acoustic wave source, changes in spectral characteristics of the tissue or material, changes in temperature, detector sensitivity changes, and electronic drifting.
Examples of acoustic type measurement systems that often require some type of calibration include acoustic spectrometers, and interferometers or any other system which uses an acoustic wave measuring instrument that outputs acoustic energy to a material and then measures some portion of the return signal.
Various types of calibration techniques and devices have been attempted. For example, U.S. Pat. No. 5,365,925 describes a calibration boot which includes a plurality of materials, which is placed over an optical catheter for the purpose of making a multi-point calibration of reflected or backscattered light. U.S. Pat. No. 5,311,273 describes a method of using four black body radiators to provide calibration of an infrared spectrometer. However, neither of these approaches involves an inexpensive calibration target that can be easily discarded after each use, and thus does not prevent a user from taking a measurement without going through a calibration step.
U.S. Pat. No. 4,981,355 describes a calibration device for the in vitro calibration of a light guide, whereby a polyethylene material has a plurality of light scattering particles and a plurality of light absorbing particles which yields a neutral density filtering type of effect, uniformly distributing light in the plastic parts of the calibrator. The calibrator can be positioned into a sterile tray which is protected by a tear off plastic cover. Once the calibration is complete, the surgeon removes the catheter from the calibrator and the tray in which it is held and then presumably disposes of the calibration device and its tray. This approach, however, is neither simple nor inexpensive.
U.S. Pat. No. 4,796,633 describes a calibration reference apparatus that fits over a light guide. A stop limits the extent to which the light guide can be advanced into the cavity whereby an endface of the light guide is spaced from a region of the surface to define a gap. The end wall and the gap are adapted to return a known ratio of the light directed into the gap from the end face of the light guide. Again, however, this approach does not involve an inexpensive, disposable calibration device.
U.S. Pat. No. 4,744,656 discloses a calibration boot that snaps into place over an optical catheter allowing calibration of the catheter before use. Once the calibration is complete, the boot is removed and the optical catheter is ready for use. Each new catheter comes with a new boot. However, the boot is not present during the measurement and there is no provision to prevent reuse of the boot.